For communication purposes, a single mode optical fiber that allows light transmission only in the dominant (HE.sub.11) mode is ideal. To produce this ideal fiber, the following relation must be satisfied: ##EQU1## wherein n.sub.1 =the refractive index of the core, a=the core radius, n.sub.2 =the refractive index of the cladding, .lambda.=the wavelength of the propagating light, .pi.=the ratio of the circumference of a circle to its diameter. The resulting fiber with the core designed to have a small diameter so as to meet this relation is called a single mode fiber and, because of the absence of pulse delay due to multimode dispersion, has great potential for use as a future high-capacity communication channel.
However, the actual "single mode fiber" has two modes, HE.sub.11.sup.(V) mode with an electric field in the vertical direction, and HE.sub.11.sup.(H) mode with a field in the horizontal direction. Furthermore, the distribution of refractive indices in a cross section is not completely coaxial (symmetric with respect to the fiber axis) but more often than not, the distribution is elliptic or draws other forms which are asymmetric with respect to the fiber axis. Because of the difference in group velocity between the two HE.sub.11 modes, pulse spread occurs at the receiving end as in the case of multimode dispersion, and this is believed to be one of the principal factors that put a limit on the transmission capacity of single mode fibers. This difference in group velocity would cause no problem if only one HE.sub.11 mode, say, its vertical mode were, excited at the transmission end and propagated down through the fiber and then, that vertical mode alone were detected at the receiving end. However, with most common single mode fibers having an axially symmetrical form, it is very rare that the front of a polarized wave in HE.sub.11 mode reaches the receiving end intact without being affected by the heterogeneity of the internal structure or fiber deformation.
Scientists have reported many methods for producing a single mode fiber that permits the front of a polarized wave to propagate through the fiber without being upset by possible deformations such as bending and torsion to which the fiber is usually subjected. According to one method, a stress providing an axially asymmetric distribution of refractive indices is applied to the interior of the fiber so that a small difference is given between the indices of refraction in two planes perpendicular to each other in the longitudinal direction of the fiber, and by so doing, a difference in phase velocity is provided between the two polarized wave components crossing at right angles and the degree of their coupling is reduced. In a refined version of this method, the core is shaped into an elliptic form or other geometries that are completely asymmetric with respect to the fiber axis, and by so doing, a difference in phase velocity is provided between two polarized wave components, one polarized in the direction of the major axis and the other being polarized in the direction of the minor axis and the degree of their coupling is also decreased. As is well known, the magnitude of coupling between two polarized components which cross at right angles is generally proportional to C/(.DELTA..beta./.beta..sub.av.) wherein C is the coefficient of coupling of the two components per unit transmission length of the fiber (C depends on the bending or torsion accompanying fiber deformations or the degree of unevenness of the internal structure of the fiber); .beta. is the difference between the phase constants of the two components; and .beta..sub.av. is the average of the two phase constants.
The essence of the two conventional methods described above is to achieve a transmission mode which is the closest to single polarization by means of providing a difference in phase velocity between two polarized components which cross with each other at right angles. However, the fact remains that both polarized waves have a transmissible mode. So, if the coupling coefficient C is great, that is, if the fiber is subjected to great deformation or if it is very long, the magnitude of the coupling between the two polarized components is increased until the condition for accomplishing single mode transmission is no longer obtainable, and this occurs even if .DELTA..beta., the difference between the two phase constants, is fairly large.
To eliminate these defects of the conventional "single mode" fiber, and for the purpose of providing an optical fiber that would theoretically be able to transmit only a single polarized, single mode, we previously invented an optical fiber that was made of a center core surrounded by a cladding having a lower refractive index than said core and which provided an even lower refractive index in two diametrically opposite portions of the core-cladding interface. We filed a patent application for this invention (Japanese Patent Application No. 102943/79) on Aug. 13, 1979. In this invention, we used a dielectric material having a relatively low dielectric constant in the selected two diametrically opposite portions of the core-cladding interface, but the specification of the invention contained no explicit reference to the degree by which the refractive index of these portions should be made smaller than that of the cladding.